Until the discovery of the "gene" it was thought that the blood carried some factor that was passed on to the offspring. Genetics, the study and understanding of how these units of heredity play a role in practically all aspects of every life form, has replaced the "blood theory". The phrase "bloodline" is so entrenched in society it is still commonly used to identify a family or lineage with a common ancestry.
In these modern times there is general knowledge that all living things inherit their characteristics through the transmission of genes. The mode of transmission may vary depending on the life form. Warm blooded animals, (which include pigeons), reproduce sexually. New offspring arise when male sperm unite with female ova. However, the details of how genes are incorporated into gametes (sperm and ova) and transmitted to the new offspring may not be so well understood by many. By learning a few fundamentals animal breeders (in this case pigeon fanciers) will greatly enhance their ability to direct their breeding efforts toward attainable goals.
To grasp how each new offspring receives its compliment of genes from each parent, one must understand the difference between the production of a normal body cell (skin, bone, etc.) and the production of sex-cells (or, gametes). The production of body or growth cells give rise to new cells with equal numbers of chromosomes. Sex-cells have half the number of chromosomes as the original cells. These half cells (gametes) join other half cells (sperm or egg) to attain the equal numbers of chromosomes as growth cells; after which, the new offspring (zygote) begins to develop. This is the mechanism that insures some variation in all living things.
Occasionally, the replication of a gene produces an altered or changed version of itself - called a mutation. Such mutations can often be identified in the phenotype of an individual. Moreover, that mutation could be transferred to future generations. Some characteristics produced by these gene mutations are precisely what breeders find so attractive. Breeders can move an identified gene which produces a desired phenotypic expression, to future generations of breeding stock - through the acquisition of a single bird.
It might be unproductive for a breeder to devote much time hoping to reproduce a particular characteristic if the desire characteristic is the results produced by more than one gene. The reason for the adoption of "wildtype" as a "standard" becomes evident. Knowledge of each gene's affect on wildtype and how to unite them in one individual offers breeders more than random hope. Without this knowledge, "good fortune" must come your way through trial and error breeding methods.
Most important to breeders is to understand the distinction between genotype and phenotype. To an uninformed breeder, stock might be discarded because it does not display any desirable characteristics. In reality, they could be eliminating stock which are possessors of the genes that produce those desired characteristics. This is particularly true when in regards to a recessive gene in a heterozygous state. Because, unlike a dominant gene, a recessive gene is generally, undetected in the phenotype.